Logarithmic amplifier

ABSTRACT

A logarithmic amplifier capable of operating with input currents as low as 10 11 amps having a capacitor feedback loop for smoothing out fluctuations at very low current inputs and a second feedback loop consisting of a resistor and diode to switch in the resistor at very low outputs to limit the time constant of the parallel connection of the feedback capacitor and logarithmic transistor elements to reduce output fluctuations at switch-on.

United States Patent References Cited UNITED STATES PATENTS May PlatzerPearlman et al..

Pearlman Embley Primary ExaminerJohn S. Heyman Assistant ExaminerDavidM. Carter Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: Alogarithmic amplifier capable of operating with input currents as low as10'" amps having a capacitor feedback loop for smoothing outfluctuations at very low current inputs and a second feedback loopconsisting of a resistor and diode to switch in the resistor at verylowoutputs to limit the time constant of the parallel connection of thefeedback capacitor and logarithmic transistor elements to reduce outputfluctuations at switch-on.

O/FFEQENWA 702 l/VPu r OPE/2A 7/0 MA 4 I 5 74 a: AMPl me i c A ll El WENT5 PATENTED SEP28 l9?! SHEET 3 [IF 3 LOGARITIIMIC AMPLIFIER Thisinvention relates to a logarithmic amplifier, and more particularly, itrelates to a logarithmic amplifier suitable for use with an ionizationchamber such as is commonly used with nuclear reactors.

Logarithmic amplifiers have in the past been built in various forms buttheir design for use with low-input currents, for example from lO'" to10 amperes, has presented a number of problems. Prior art amplifiershave commonly needed ovens to maintain at least some of their componentsat constant temperature for stability, moreover, they have requiredlarge numbers of adjustments and trims in use; and they have typicallygiven rise to noise and drift problems.

Accordingly, it is an object of the present invention to provide alogarithmic amplifier suitable for use with an input current rangingfrom approximately 10" to l amperes, which amplifier requires little ifany adjustment in use, and in which no oven is required, and in whichnoise and drift problems are reduced.

Further objects and advantages of the invention will appear from thefollowing disclosure, in which the embodiment discussed is presented forillustrative purposes, the scope of the invention being definedprimarily by the appended claims.

ln the drawings:

FIG. 1 is a schematic, partly in block diagram form, of a logarithmicamplifier according to the present invention;

FIG. 2 illustrates various voltage-current curves for portions of thecircuit of FlG. 1;

FIG. 3 is a more detailed circuit representation of the device of FIG.1;

FIG. 4 shows output voltage curves for the circuit of FIGS. 1 and 3; and

FIG. 5 shows a modification'of the circuit of FIG. 1.

Referring firstly to FIG. 1, there is shown a schematic, partly in blockform, of a logarithmic amplifier in accordance with the presentinvention. An input current i from an ionization chamber showndiagrammatically at l is directed over a cable 2 to the input terminal 3of an insulated gate field effect transistor input stage 4. The outputsignal from the input stage 4 is directed to a conventional operationalamplifier 6, to which is connected a conventional stabilizing network 8.The output from the operational amplifier 6 is next directed to acascode follower output amplifier 10.

Output amplifier 10 provides a low impedance output into an output cable12 to a differentiator shown in block form at 14. The differentiator 14is commonly used in conjunction with nuclear reactors to differentiatethe output signal e, from the logarithmic amplifier and to shut down thereactor as a safety measure when such output signal (which is a measureof the current from the ionization chamber 1) increases too rapidly. Thecascode follower output amplifier 10 is used to provide a low-outputimpedance because the output cable 12 will often be relatively long,with consequent high capacitance, and the differentiator 14 willnormally include a series capacitance. The output signal must be able tocharge the capacitances quickly in order to avoid errors and lags in theresponse of the differentiator l4, and thus a low-output impedance forthe logarithmic amplifier is needed.

A feedback loop from the output amplifier 10 is provided, through theseries combination of an attenuator 16 (containing resistors R15 and R16in series, resistor R16 being adjustable), a temperature driftcompensation network generally indicated at 18, and logarithmic elementsgenerally indicated at 20, back to the input terminal 3 of the fieldeffect transistor input stage 4. The logarithmic elements 20 consist ofthe series-connected emitter-base junction diodes of two silicon planartransistors Q1 and Q2. The collectors of transistors 01 and 02 are tiedto their respective bases to reduce the effect of the body resistance ofthe base regions of these transistors.

The temperature compensation network 18 includes a pair of transistorsQ3 and Q4 having the same characteristics as transistors 01 and Q2 butconnected in opposition thereto. The emitter currents of transistors Q3and Q4 are set so that the averageemitter currentisapproximately equalto the maximum current expected through the base-emitter diodes oftransistors Q1 and Q2. Transistors Q3 and Q4 thus serve to cancel theconstant current" temperature drift of transistors Q1 and Q2. Atemperature compensation resistor R1 is provided to compensate for thevariation in temperature characteristics of transistors Q1 and 02 withrespect to the current flowing through these transistors. A, B, C and Dare reference points.

FIG. 2 illustrates the principles of the temperature compensationnetwork 18 in more detail. FIG. 2 plots voltage against the logarithm ofthe current through the feedback loop, with reference to the variouspoints A, B, C, and D of FIG. I. In FIG. 2A, curve 1 plots the voltagebetween points A and B at one temperature, e.g., 25 C., while curve 2plots the voltage between points A and B at another higher temperature,e.g., 50 C. It will be noted that a change in temperature causes thevoltage characteristics of transistors Q1 and O2 to shift, and that atwo-component correction will bring the two curves 1 and 2 back intocoincidence (so that there will be no variation with temperature). Thefirst correction needed is a translation of the curves so that they willintersect near the current range in question, and the second correctionneeded is a rotation about the intersection point (change in slope) ofone of the curves so that the two curves will then coincide.

The first correction component is supplied by transistors Q3 and Q4.FIG. 2B plots voltage between points B and C as a function of theemitter currents of transistors Q3 and Q4, curve 3 being the 25 C. lineand curve 4 being the 50 C. line, and the emitter currents oftransistors Q3 and Q4 being set at the value indicated by the dottedline of FIG. 2B. To obtain the voltage between points A and C as afunction of current through transistors Q1 and Q2, the voltage values ofcurves 3 and 4 at the fixed emitter currents of transistors Q3 and Q4are added to curves 1 and 2 respectively, resulting in curves 5 and 6respectively of FIG. 2C. Curves 7 and 8 of FIG. 2C are similar to curves5 and 6 but show the voltage between points A and D instead of points Aand C, curve 8 being plotted in the absence of the effect of temperaturecompensationresistor R1.

When the effect of resistor R1 is taken into account, curve 8 isrotated, i.e., its slope is changed, with the new result being shown ascurve 8' in FIG. 2C. Thus, resistor R1 supplies the second correctioncomponent needed for temperature compensation.

The feedback network of the logarithmic amplifier illustrated in FlG. 1also includes a time-constant capacitor C 1 shown connected across thelogarithmic elements Q1 and Q2. Capacitor CI could of course beconnected directly between the output of cascode follower amplifier 10and terminal 3, but location of capacitor C1 as shown is more convenientfrom the point of view of routine construction and operation of thecircuit.

The reason for the presence of time constant capacitor C1 is as follows.When the output current from the ionization chamber 1 is at a low level,for example, 10" amperes, then normal statistical variations in theionization measured in the ionization chamber will give rise to widefluctuations in the current i therefrom. if such fluctuations in thecurrent i were not smoothed the differentiator 14 would trigger safetydevices (not shown) unnecessarily often, and the nuclear reactor (notshown) would be shut down at undesired times. Therefore, thefluctuations of current i at low current levels are averaged or filteredwith the smoothing circuit formed by capacitor C1 and the resistance oftransistors Q1 and 02. At higher current levels, the statisticalfluctuations in current i are much smaller in relation to the averagecurrent, so that little filtering is needed. However, less filteringoccurs at higher levels because the resistance of logarithmic elementsQ1 and Q2 drops greatly with an increase in current through theseelements. If the resistance of the series base-emitter diodes oftransistors 01 and O2 is denoted by R, then resistance R decreases asthe current through transistors Q1 and Q2 increases, and hence the timeconstant RC1 of the filter constituted by the combination of transistorsQ1 and Q2, and capacitor C 1, becomes smaller. At relatively high-inputcurrents, the resistance R of the logarithmic elements Q1 and O2 is solow that the effect of capacitor C1 is of little significance.

Although capacitor C1 is primarily intended as a smoothing capacitor,providing, with logarithmic elements Q1 and Q2 a variable (with inputcurrent) time constant, the choice of value for capacitor C1 is alsoaffected by another factor. In practice, the input cable 2 from theionization chamber 1 will normally be relatively long and hence willpossess considerable capacity and it is found that the higher thiscapacity, the more high frequency amplifier noise there will be at theoutput of the logarithmic amplifier. However, it is also found that ahigher value for feedback capacitor C1 assists in reducing the amplifiernoise in the output signal.

Referring again to FIG. 1, it will be noted that there is provided afeedback path consisting of a diode D1, and resistors R2 and R3.

The purpose of this arrangement is to compensate for the followingphenomenon: if the nuclear reactor (not shown) generating the ionizationcurrent has been shut down, the input current i will be very small. Aspreviously mentioned, at very low signal currents the resistance oflogarithmic elements Q1 and Q2 becomes extremely high. Then, after thereactor is restarted, the input current i increases and in the absenceof resistor R3, would charge capacitor C1 for a period of timedetermined by time constant RC1, where R is as mentioned the equivalentresistance of logarithmic elements 01 and Q2. Since R is large at thistime, an undesirably long period of time would be required for theoutput of the logarithmic amplifier to reach equilibrium. If a fixedresistance having a lower value than that of logarithmic elements 01 andQ2 were connected across capacitor C1 during the low-input currentstages, then capacitor C1 would be charged by the input current to avoltage limited by this lower resistance and hence, upon restoration ofthe signal from the ionization chamber, would reach equilibrium muchsooner. Such lower resistance is provided by resistor R3 which isconnected into the feedback circuit when diode D1 becomes forwardbiased, this occurring when the output signal at cable 12 becomessufficiently low.

In the exemplary circuit shown in FIG. 1 (and illustrated in more detailin FIG. 3, to be referred to shortly) the output voltage e at cable 12varies from volts at full scale input current (approximately amperes) to0 volts at low-input currents (l0 amperes). At input currents below 10amperes, the output voltage e, begins to go positive, i.e., reversessign. Diode D1 then becomes forward biased and connects resistor R3across capacitor C1 as mentioned. When the output voltage e begins tocome back on scale, i.e., decreases below zero volts, diode D1 becomesreverse biased and disconnects resistor R3, so that logarithmic feedbackthrough elements Q1 and O2 is restored.

Resistor R3 may be thought of as a maximum time constant limitingresistor, limiting the maximum time constant of the feedback path to aselected value at times when the resistance R of logarithmic elements Q1and Q2 approaches infinity. The switching-in of resistor R3 at verylow-input currents has the further advantage that it reduces suddenjumps in the output voltage e that would otherwise tend to occur(because of the nonnal circuit characteristics of the logarithmicelements, Q1 and Q2, and the capacitor C1, in parallel) during theinitial turn-on period. Sudden jumps in the voltage e,, are undesirableexcept where they indicate adanger condition in the reactor, becausesuch sudden jumps will cause the differentiator 14 to provide a largeoutput to trigger safety devices (not shown) to shut down the reactor.

The actual time constant of the feedback loop at times when diode D1 isforward biased and the resistance R of elements 01 and Q2 approachesinfinity, may be shown to be C1R1R3/(R1+R15+R16). This assumes that thegain of operational amplifier 6 is very high, so that input terminal 3(reference point A) is very nearly at ground potential. It may be notedthat the resistor R3, instead of being connected as shown, could beconnected across capacitor C1 (with its value changed to the fractionR1/(R1+R15+R16) of its previous value, in order to maintain the sametime constant). However, R3 is placed as shown, so that as fewcomponents as possible are utilized at very low currents, and so that aslarge a switching voltage as possible is available for switching diodeD1.

A more detailed schematic of the logarithmic amplifier of FIG. 1 ispresented in FIG. 3. In FIG. 3, the input amplifier 4 is shown asincluding two insulated gate field effect transistors 05 and Q6connected in a differential amplifier configuration. These transistorsare of the p-channel type. The input cable 2 is connected through aninput resistance R4 to the gate of transistor Q5, the gate of transistorQ6 being grounded. The source terminals of transistors Q5, Q6 areconnected together through a balancing potentiometer R5, the tap ofwhich is connected through a resistance R6 to a supply voltage +v., Thedrain terminals of transistors Q5 and Q6 are connected through resistorsR7 and R8 to a supply voltage -v., and are also connected to inputterminals 20 and 22 of the operational amplifier 6, which is shown as adifferential amplifier in FIG. 3.

It will be appreciated that the input amplifier 4 need not be adifferential amplifier as shown, but use ofa differential inputamplifier is convenient because in this way, the temperature drift inone field effect transistor O5 is cancelled by the temperature drift inthe other field effect transistor Q6. In addition, extra gain isachieved by providing differential inputs into the operational amplifier6.

The operational amplifier 6 includes a stabilizing network consisting ofcapacitors C2, C3 and C4 and resistors R9 and R10. This network, whichis conventional in design, stabilizes the operational amplifier overlarge changes in feedback due to the changes in resistance of thelogarithmic elements Q1 and Q2. Power for the operational amplifier 6 isprovided from terminal 24, which is connected to a zener diode Z1leading to ground, and to a resistor R11 connected to the v. supply.Power for the amplifier 6 is also provided from a terminal 26 which isconnected to a zener diode Z2 connected to ground and to a resistor R12connected to the +v. supply.

The operational amplifier 6 includes an output terminal 28 connected tothe input of the cascode follower output amplifier 10, which is shown asincluding transistors Q7 and Q8. The collector of transistor O7 isconnected to the base of transistor Q8 and is also connected through aresistor R13 to the -v. supply, while the emitter of transistor Q7 isconnected to the collector of transistor Q8. The emitter of transistorQ8 is connected through an inductor L1 to the v. supply and through acapacitor C5 to ground.

The output signal from output amplifier 10 is taken at the emitter oftransistor 07. This output is directed to cable 12 and is also directedthrough diode D1, and through resistor R3 when diode D1 is turned on, tothe collector of logarithmic element 01. Resistor R2 enables diode D1 tobecome forward biased when the output signal e begins to go positive.

Connected to the emitter of transistor O7 is a voltage divider chainconsisting of zener diode Z3 and resistors R14, R15, R16 and R1. Zenerdiode Z3, which is connected between the emitter of transistor Q5 andresistor R14 (R14 in turn being connected to the +v. supply) provides apotential rise, i.e., in effect acts as a simple battery, in order thatfull scale output voltage will be obtained at the maximum input currentand zero output voltage will be obtained at the minimum input current.The effect of zener diode Z3 is shown in FIG. 4, which plots the voltagebetween points A and E, point E being the point at which the outputvoltage is taken. (It will be recalled that point A is virtually atground potential). Output voltage curves 9 and 10 appear in FIG. 4, andthese curves are the same as curves 7 and 8 respectively of FIG. 2C, butwith the latter curves shifted upwardly by virtue of zener diode Z3, sothat maximum and minimum output voltages will occur at maximum andminimum input currents respectively.

Resistor R14 acts primarily as a load for transistor Q7, while resistorsR15 and R16 act, as mentioned, as an attenuator to assist in controllingthe amount of feedback around the feedback loop. In particular, resistorRl6 is used to set the desired span of measuring current range. ResistorR1 compensates for some of the temperature drift in logarithmic elementsQ1 and Q2, as discussed.

The junction between resistors R16 and R1 is connected to the base oftransistor Q3, and the emitter of transistor Q3 is connected in turn tothe base of transistor Q4, the collectors of both these transistorsbeing connected to the cathode of Zener diode Z4 which controls thecollector voltage of transistors Q3 and Q4. Zener diode Z4 is connectedbetween ground and a resistor R17 which is connected to the +v. supply.The emitters of transistors Q3 and Q4 are connected through resistorsR18 and R19 to the v. supply. The emitter of transistor Q4 is alsoconnected to the emitter of logarithmic element Q2, the base andcollector of logarithmic element Q2 being connected in turn to theemitter of logarithmic element Q1, and the base and collector oflogarithmic element Q1 being connected to input terminal 3.

In practice, the circuit shown in FIG. 3 has been found to operatesatisfactorily with the following component values and types, thetransistor and diode types being listed in standard notation:

Transistors Diodes DI F0300 Resistor! Rl 2K temperaturecompensatingresistor R2, R4, Rl3 10K R7, R8 26K Rl l 820 ohms Rl2 220 ohms R17 680ohms RIB, R19 selected for full scale input current Capacitors CI 4,700r.

C2 1.0 ,tr.

C5 L200 f.

Inductor LI 1 mh.

Amplifier l Fairchild-ty e WA702 DC amplifier Voltages +V and V +l 5volts and l5 volts With the values as noted above, the logarithmicamplifier described covers the range from to about 10 ampercs inputcurrent, with the output voltage e, at cable 12 ranging from 0 volts atthe low-range input current to 5 volts at about 10 amperes current.

With the circuit values shown, the drain currents for the field effectinput transistors Q5 and Q6 were held at about 0.5 milliamperes, i.e.,to a value at which the temperature drift in characteristics of thesetransistors was low.

It may be noted that more than two logarithmic elements in series couldbe used, and this would naturally result in a higher voltage across thelogarithmic elements, which in a sense is desirable since this tends toswamp the drift in the input amplifier 4. However, it is found that withtoo many logarithmic elements in series, their impedance becomes high,particularly at low-input currents, and it is then found that the timeconstant constituted by the product of the resistance of the serieslogarithmic elements and the capacitance of capacitor C 1 becomes toohigh, degrading the response of the device. The value of capacitor C1must therefore be lowered, but this causes an undesirably high degree ofamplifier output noise when a long input cable 2 from the ionizationchamber 1 is used. It is therefore found that two logarithmic elementsin series provide a good compromise and lead to efficient operation ofthe circuit.

If desired, a low current correction facility may be built into thedevice, as shown in FIG. 5. In FIG. 5, a transistor 09 has beeninserted, biased from a potentiometer R20, to add a very low correctioncurrent to the current flowing through logarithmic elements Q1, and Q2,to improve their linearity at values of input current i towards thelower end of the range.

We claim:

1. A logarithmic amplifier for providing an output signal proportionalto the logarithm of an input current signal within a range extendingdown to approximately 10" amps, said amplifier comprising:

a. signal input means for said input signal,

b. amplifier means having amplifier input means, and having amplifieroutput means for said output signal,

c. said amplifier input means being connected to said signal inputmeans,

d. logarithmic element feedback means coupled between said amplifieroutput means and said signal input means,

e. capacitor feedback means coupled between said amplifier output meansand said signal input means for smoothing out input signal fluctuationsat the lower end of said range,

f. and a passive circuit consisting of a combination of i. a maximumtime constant limiting resistance ii. and switch means operative inresponse to a predetermined level of said output signal corresponding toan input signal at the extreme lower end of said range,

g. said passive circuit being coupled between said amplifier outputmeans and said signal input means, said switch means being operative toinsert said time constant limiting resistance in circuit between saidamplifier output means and said signal input means to limit the timeconstant of the parallel connection of said logarithmic element feedbackmeans and said capacitor feedback means upon occurrence of saidpredetermined level of said output signal.

2. A logarithmic amplifier according to claim 1 wherein said logarithmicelement feedback means comprises a pair only of logarithmic elements,said logarithmic elements being transistors with the emitter-basejunctions of said transistors connected in series, and each of saidtransistors having its collector connected to its base.

3. A logarithmic amplifier according to claim 2 including temperaturecompensation means connected in series with said pair of transistors,said temperature compensation means comprising:

h. a further pair of transistors having their emitter-base junctionsconnected in series with the emitter-base junctions of said firstmentioned pair of transistors to provide a first temperature dependentcompensating variation in feedback between said amplifier output meansand said signal input means (a), and

i. temperature dependent resistance means connected to said further pairof transistors to provide a second temperature dependent compensatingvariation in feedback between said amplifier output means and saidsignal input means (a).

4. A logarithmic amplifier according to claim 3 wherein said signalinput means (a) is a differential amplifier including a pair ofinsulated gate field effect transistors as input elements, said inputsignal being applied to the gate of one of said field effecttransistors, and the gate of the other of said field effect transistorsbeing grounded.

5. A logarithmic amplifier for providing an output voltage proportionalto the logarithm of an input current over a range of input currentsvarying from a predetermined low value at least as low as approximatelyl amps to values greater than said predetermined low value, said outputvoltage having a predetermined level when said input current has saidlow value, said amplifier comprising:

a. signal input means for said input current and including an insulatedgate field effect transistor as a current input element,

b. an operational amplifier connected to said signal input means,

c. a low impedance output amplifier connected to said operationalamplifier and having output means for said output voltage,

d. feedback means connected between the output means of said outputamplifier and said signal input means and comprising:

i. attenuator and temperature compensation means connected to saidoutput means,

ii. a pair of logarithmic elements connected in series between saidmeans (d) (i) and said signal input means,

iii. a time constant capacitor connected in parallel with saidlogarithmic elements for smoothing out input current fluctuations atsaid low value,

iv. diode switch means connected to said output means and operative inresponse to occurrence of said predetermined level of said outputvoltage,

v. and a time constant limiting resistance connected between said diodeswitch means and said signal input means, said diode switch means beingoperative to connect said resistance between said output means and saidsignal input means to limit the time constant of the parallel connectionof said logarithmic elements and said time constant capacitor uponoccurrence of said predetermined level of said output voltage.

6. A logarithmic amplifier according to claim 5 wherein e. said pair oflogarithmic elements consist of first and second planar transistors, thecollector and base of said first transistor being connected to saidsignal input means (a) and the collector and base of said secondtransistor being connected to the emitter of said first transistor,

f. and said temperature compensation means includes i. third and fourthtransistors of the same type as said first and second transistors, theemitter of said third transistor being connected to the emitter of saidsecond transistor and the emitter of said fourth transistor beingconnected to the base of said third transistor, said third and fourthtransistors providing a first temperature dependent variation infeedback around said feedback means,

ii. and temperature sensitive resistance means connected to the base ofsaid fourth transistor to provide a second temperature dependentcompensating variation in feedback around said feedback means.

1. A logarithmic amplifier for providing an output signal proportionalto the logarithm of an input current signal within a range extendingdown to approximately 10 11 amps, said amplifier comprising: a. signalinput means for said input signal, b. amplifier means having amplifierinput means, and having amplifier output means for said output signal,c. said amplifier input means being connected to said signal inputmeans, d. logarithmic element feedback means coupled between saidamplifier output means and saId signal input means, e. capacitorfeedback means coupled between said amplifier output means and saidsignal input means for smoothing out input signal fluctuations at thelower end of said range, f. and a passive circuit consisting of acombination of i. a maximum time constant limiting resistance ii. andswitch means operative in response to a predetermined level of saidoutput signal corresponding to an input signal at the extreme lower endof said range, g. said passive circuit being coupled between saidamplifier output means and said signal input means, said switch meansbeing operative to insert said time constant limiting resistance incircuit between said amplifier output means and said signal input meansto limit the time constant of the parallel connection of saidlogarithmic element feedback means and said capacitor feedback meansupon occurrence of said predetermined level of said output signal.
 2. Alogarithmic amplifier according to claim 1 wherein said logarithmicelement feedback means comprises a pair only of logarithmic elements,said logarithmic elements being transistors with the emitter-basejunctions of said transistors connected in series, and each of saidtransistors having its collector connected to its base.
 3. A logarithmicamplifier according to claim 2 including temperature compensation meansconnected in series with said pair of transistors, said temperaturecompensation means comprising: h. a further pair of transistors havingtheir emitter-base junctions connected in series with the emitter-basejunctions of said first mentioned pair of transistors to provide a firsttemperature dependent compensating variation in feedback between saidamplifier output means and said signal input means (a), and i.temperature dependent resistance means connected to said further pair oftransistors to provide a second temperature dependent compensatingvariation in feedback between said amplifier output means and saidsignal input means (a).
 4. A logarithmic amplifier according to claim 3wherein said signal input means (a) is a differential amplifierincluding a pair of insulated gate field effect transistors as inputelements, said input signal being applied to the gate of one of saidfield effect transistors, and the gate of the other of said field effecttransistors being grounded.
 5. A logarithmic amplifier for providing anoutput voltage proportional to the logarithm of an input current over arange of input currents varying from a predetermined low value at leastas low as approximately 10 11 amps to values greater than saidpredetermined low value, said output voltage having a predeterminedlevel when said input current has said low value, said amplifiercomprising: a. signal input means for said input current and includingan insulated gate field effect transistor as a current input element, b.an operational amplifier connected to said signal input means, c. a lowimpedance output amplifier connected to said operational amplifier andhaving output means for said output voltage, d. feedback means connectedbetween the output means of said output amplifier and said signal inputmeans and comprising: i. attenuator and temperature compensation meansconnected to said output means, ii. a pair of logarithmic elementsconnected in series between said means (d) (i) and said signal inputmeans, iii. a time constant capacitor connected in parallel with saidlogarithmic elements for smoothing out input current fluctuations atsaid low value, iv. diode switch means connected to said output meansand operative in response to occurrence of said predetermined level ofsaid output voltage, v. and a time constant limiting resistanceconnected between said diode switch means and said signal input means,said diode switch means being operative to connect said resistancebetween said output means and said signal input means to limit the timeconstant of the parallel connEction of said logarithmic elements andsaid time constant capacitor upon occurrence of said predetermined levelof said output voltage.
 6. A logarithmic amplifier according to claim 5wherein e. said pair of logarithmic elements consist of first and secondplanar transistors, the collector and base of said first transistorbeing connected to said signal input means (a) and the collector andbase of said second transistor being connected to the emitter of saidfirst transistor, f. and said temperature compensation means includes i.third and fourth transistors of the same type as said first and secondtransistors, the emitter of said third transistor being connected to theemitter of said second transistor and the emitter of said fourthtransistor being connected to the base of said third transistor, saidthird and fourth transistors providing a first temperature dependentvariation in feedback around said feedback means, ii. and temperaturesensitive resistance means connected to the base of said fourthtransistor to provide a second temperature dependent compensatingvariation in feedback around said feedback means.